First of all, a big thank you all for advice and assistance!
Background tl;dr
My Xperia X is stuck on 4.4.0.58 as lack of space on the root partition prevents further upgrades.
Game plan
I have decided to do a fresh install of Sailfish OS once I have migrated important data to a new device. The primary reason for this is that I’ve had a number of failed OS upgrades/command line recoveries since 2018, plus uncountable apps installed, tested and removed. The idea now being that starting over will remove any cruft and potential interference from various packages from various sources.
Before wiping the X I will use it as a playground for trying out the good suggestions I have received in this thread. I’ll start by addressing storage space, then go on to extend the root partition.